Sophie Amundsen is the main character in Sophie's World. She is the product of Albert Knag, an author whose book Sophie's World was written for his daughter Hilde Knag. Although she is only a figment of his imagination, she manages by the end of the novel to escape her nonexistence and enter the real world, albeit as a spirit.
Sophie's 15th birthday was June 15, the same day as Hilde's. Throughout the book Sophie befriends Alberto Knox, another creation of Albert Knag. Early on she lived a normal life, but after receiving a letter asking her questions such as "Who are you?", she begins to ponder. Sophie takes a deep interest in philosophy, in contrast to other characters in her world, such as her mom who initially finds it absurd.
As the lessons progress Sophie becomes incredibly knowledgeable. She remembers everything that Alberto teaches her and she begins to synthesize her own theories of life, and generally she begins to see life through a new point of view. Using her education, she understands and manages to cope with the real world. In fact, she boldly begins to believe she can affect Hilde's world. Alberto disagrees with her, but Sophie asserts her position, and she manages a slight interaction with Hilde when she hits her with a wrench.